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    public class SectionsSchedules
    {
        [Key]
        public int Id { get; set; }
        [ForeignKey("Disciples_Id")]
        public int? Disciples_Id { get; set; }
        public Schedule Disciples { get; set; }
    }

    public class Schedule
    {
        [Key]
        public int Id { get; set; }
        public int SectionsSchedules_Id { get; set; }
        [ForeignKey("SectionsSchedules_Id")]
        public SectionsSchedules Section { get; set; }
    }

            modelBuilder.Entity<SectionsSchedules>()
            .HasOptional(p => p.Disciples)
            .WithRequired(p => p.Section).WillCascadeOnDelete(true);

"One or more validation errors were detected during model generation:\r\n\r\nSectionsSchedules_Disciples_Target: : Multiplicity is not valid in Role 'SectionsSchedules_Disciples_Target' in relationship 'SectionsSchedules_Disciples'. Because the Dependent Role properties are not the key properties, the upper bound of the multiplicity of the Dependent Role must be

[ForeignKey("Disciples_Id")]
public int? Disciples_Id { get; set; }